The application of a coating by electrodeposition involves depositing a film-forming composition to an electrically conductive substrate under the influence of an applied electrical potential. Electrodeposition has gained prominence in the coatings industry because in comparison with nonelectrophoretic coating methods, electrodeposition provides higher paint utilization, outstanding corrosion resistance and low environmental contamination. Early attempts at commercial electrodeposition processes used anionic electrodeposition where the workpiece being coated served as the anode. However, in 1972 cationic electrodeposition was introduced commercially. Since that time cationic electrodeposition has become increasingly popular and today it is the most prevalent method of electrodeposition. For instance, cationic electrodeposition is the process of choice for applying a primer coating in the manufacture of motor vehicles throughout the world.
Many cationic electrodeposition coating compositions used today are based on active hydrogen-containing resins derived from a polyepoxide and a capped polyisocyanate curing agent. These cationic electrodeposition compositions conventionally contain organotin catalysts, such as dibutyl tin oxide and lead catalysts, to activate cure of the electrodeposition coating composition. Because of cost and environmental considerations, the levels of these catalysts are kept low. However, low catalyst levels may lessen the cure response of a coating composition providing weaker properties in the cured film than desired. Appearance of the cured film may also be adversely affected.
Schipfer et al. in South African Patent Application No. 93/2977 and U.S. Pat. No. 5,507,928 (Bohmert et al.) disclose the use of cationic electrodepositable coating compositions which contain catalysts that are specific complexes or salts of bismuth and carboxylic acids, in particular hydroxycarboxylic acids. As noted in these references, these catalysts are unlike available bismuth salts of relatively long-chain acids and inorganic bismuth compounds. The disclosed catalysts of these patent documents can displace the use of lead and tin compounds as catalysts for coatings in electrodeposition. The resulting electrodeposited coatings with the use of the disclosed bismuth salts reportedly have excellent application and film properties.
Unfortunately, the industrial development of bismuth salts as described above have encountered some difficulty as noted in Patent Cooperation Treaty publication no. WO95/07377, published Mar. 16, 1995. This publication notes that two disadvantages were encountered on an industrial scale for the use of the bismuth carboxylic acid salts of the aforementioned reference. One disadvantage is a tendency of the isolated and dried bismuth salts to agglomerate during storage. A second disadvantage is a necessity for the use of a greater amount of acid to digest the bismuth oxide than is required for neutralization of the paint binder resin in the electrodepositable coating composition. The invention of the WO95/07377 publication confronts these disadvantages by preparing the compositions containing the bismuth-carboxylic acid salt in a stepwise process to produce a mixture of bismuthyl lactate and bismuth lactate. The mixture is combined with the cationic paint binder to result in a bismuth content on a total solids basis of 25 to 45 weight percent for such a combined composition.
The aforementioned latter problem of digestion of the bismuth oxide by greater amounts of acid can engender an additional difficulty of a low pH of the resulting electrodeposition coating bath. Low pH for the bath of the electrodeposition coating composition generally results in more aggressive dissolution of iron from any number of sources. Several possible sources include equipment of the electrodeposition coating bath system such as that for agitation like pumps and piping that are constructed from mild steel. Also, the substrates to be electrocoated may on occasion remain in the treatment bath for some period of time without applied voltage. Soluble iron from such sources can be detrimental to the appearance of the electrocoated substrate and to the stability of the electrocoating bath.
It would be desirable to provide a curable electrodepositable coating composition which demonstrates acceptable cure response without loss of cured film properties or appearance and which contains catalysts that do not have the shortcomings of lead-type catalysts used in the art and that result in a suitable pH for the bath of the electrodepositable coating composition.